Mmusi Maimane says Jacob Zuma is showing contempt for Parliament and the Constitution.
Mthembu was shot in the face while withdrawing money from an ATM in Emalahleni last week.
The hearings are taking place in the absence of EFF leader Julius Malema and his fellow MPs.
De Lille said recent protests in parts of the city are a major stumbling block for service delivery.
The ANC says Numsa is wrong to claim that it has deviated from the Freedom Charter.
The party's representatives on the committee stormed out soon after the hearing resumed.